This invention relates to a circuit arrangement for operating a high-pressure discharge lamp in conjunction with a controlled current limiter by means of a switching signal produced in the circuit arrangement and resulting from at least a first comparison of a lamp-dependent control signal S with a reference signal. The control signal S is at least composed of a summation of a lamp-voltage-dependent part and a lamp-current-dependent part. The invention further relates to a device provided with the circuit arrangement and to a lamp provided with the circuit arrangement.
A circuit arrangement of the kind mentioned in the opening paragraph is known from U.K. Patent Specification 1,167,920.
The known circuit arrangement is connected to two thyristors arranged in parallel with opposite polarities as a controlled current limiter. A coil operative as a current stabilization ballast is connected in series with the thyristors. The anti-parallel connected thyristors may be replaced by a triac. However, it is alternatively possible that the combination of thyristors and current stabilization ballast be replaced as a whole by a controlled current limiter.
It is common practice for high-pressure discharge lamps to be operated with an alternating voltage or with a pulsatory direct voltage. The power at which the lamp is operated is to be understood here to mean the power averaged over a time which is long as compared with the period of the alternating voltage and the pulse voltage, respectively. An average lamp voltage and current, respectively, may be formed by averaging in time the absolute value of the lamp voltage and lamp current, respectively. Another way in which an average lamp voltage and lamp current, respectively, may be formed is by the root of the time average of the square of the lamp voltage and current, respectively, the so-called R.M.S. value. In each period of the alternating voltage, the actual lamp voltage waveform will include a time period of comparatively very low voltage value, a re-ignition peak voltage and a time period having a comparatively high and approximately constant voltage value. The comparatively high approximately constant value is known under the designation of plateau voltage and its time duration corresponds to the time duration in which a discharge arc occurs.
In the known circuit arrangement, a high-pressure discharge lamp can be operated at a substantially constant power. For this purpose, at a nominal value of the lamp current and a nominal value of the lamp voltage and lamp-current-dependent part of the control signal is chosen to be equally as large as the lamp-voltage-dependent part. For a lamp with a work-point in the proximity of the nominal values of the average lamp voltage and the average lamp current, the control signal thus summed forms a very close approximation to a control procedure according to the product of lamp voltage and lamp current. A circuit arrangement in which signals are subjected to an addition can be practically realized in a considerably simpler manner than a circuit arrangement in which a multiplication of signals is effected.
High-pressure discharge lamps, more particularly high-pressure sodium discharge lamps, form very efficient light sources which are frequently used. A general phenomenon, especially of high-pressure sodium discharge lamps, is that during the lamp life time the lamp voltage varies. This influences not only the power consumed by the lamp and the intensity of the luminous flux emitted by the lamp, but also, as has been found, the color temperature T.sub.c of the light emitted by the lamp.